(Table 1) Interstitial water geochemistry at DSDP Leg 84 Holes

On DSDP Leg 84, drilling was conducted at three gas-hydrate-bearing sites on the Middle America Trench slope off Costa Rica (Site 565) and off Guatemala (Sites 568 and 570). At Site 569, on the mid-slope off Guatemala, hydrates may be present, according to the seismic profile (GUA-13), although the pore-water composition does not provide clear evidence. Sites 566 and 567, on the lower Guatemala Trench slope, appear to be free of hydrates, except in fractures of serpentinite at the bottom of Hole 566.
Hydrate-bearing Sites 565, 568, and 570 show the effects of hydrate decomposition on pore-water chemistry that have been established during previous drilling at Sites 496 and 497 on the Guatemala Trench slope. These include a chlorinity decrease and d18O increase downsection. The new results, however, reveal more complex relationships between the chlorinity decrease and d18O increase than previously recognized. At Site 565, d18O values decrease in the middle section of the hole, whereas chlorinity continues to decrease from the top to near the bottom of the hole. Early diagenetic alteration of volcanic glass is suggested as a mechanism for the unexpected minimum in the O-isotope curve. Multiple fractionation by the pore-water/hydrate system is required to explain d18O-values greater than 2.7 per mil at the bottom of Hole 568, because with a fractionation factor of alpha = 1.0027, this is the maximum figure a single-stage fractionation could produce.
In situ water samples from hydrate zones in most cases failed to display the elevated salinities expected for the residual pore waters not involved in hydrate formation. This is probably because the in situ sampling device still allows a systematic pressure drop sufficient to trigger hydrate decomposition in the immediate vicinity of the sample port.

在深海钻探计划（Deep Sea Drilling Project, DSDP）第84航次中，科研团队于哥斯达黎加外海中美洲海沟（Middle America Trench）斜坡的565站位，以及危地马拉外海中美洲海沟斜坡的568、570站位这3处含天然气水合物（gas hydrate）站位开展了钻探作业。位于危地马拉外海中斜坡的569站位，虽通过地震剖面（seismic profile, GUA-13）推测可能赋存天然气水合物，但孔隙水（pore water）组分未提供明确佐证。而地处危地马拉海沟下斜坡的566、567站位，则基本不含天然气水合物，仅566号钻孔底部的蛇纹岩（serpentinite）裂隙中存在少量水合物。
含天然气水合物的565、568及570站位，呈现出与危地马拉海沟斜坡496、497站位既往钻探结果一致的水合物分解对孔隙水化学特征的影响，具体表现为随剖面深度增加，孔隙水氯度（chlorinity）降低、δ¹⁸O（原文标注为d18O）值升高。但本次新研究结果显示，氯度降低与δ¹⁸O值升高之间的关联，较此前认知更为复杂。在565站位，钻孔中段的δ¹⁸O值反而出现下降，而氯度则从钻孔顶部持续降低至近孔底位置。研究推测，火山玻璃（volcanic glass）的早期成岩蚀变（diagenetic alteration），是导致氧同位素曲线（O-isotope curve）出现异常低值的成因机制。568号钻孔底部的δ¹⁸O值超过2.7‰（per mil），要解释这一结果，需考虑孔隙水-水合物体系的多次分馏作用：当分馏系数（fractionation factor）α=1.0027时，单阶段分馏所能产生的最大δ¹⁸O值仅为2.7‰。
多数情况下，采自水合物赋存带的原位水样，并未呈现出未参与水合物形成的残余孔隙水所应有的盐度升高特征。该现象大概率源于原位采样装置（in situ sampling device）仍会产生系统性压降（pressure drop），足以在采样口附近触发水合物分解。